The present invention relates to semiconductor manufacturing technology, and more particularly to a method for selectively oxidizing a silicon wafer used in the semiconductor manufacturing technology.
When forming semiconductor devices, such as a field effect transistor called FET, on a silicon wafer, prior to manufacture of the devices, generally, isolation regions consisting of silicon dioxide are formed on the surface of the wafer to delineate active regions where devices are formed.
One of the methods by which to form isolation regions is the LOCOS process. In this LOCOS process, the surface on one side of the wafer is partially covered by a silicon nitride film with interposition of a pad oxide film. Under this condition, the exposed regions of the above-mentioned surface are oxidized to form isolation regions.
To carry out the above-mentioned LOCOS process effectively, a number of silicon wafers are held standing up on their edges or lying horizontal mutually spaced apart on a holder called a wafer boat and are transferred into an oxidizing atmosphere, in which a pad oxide film is formed on both surfaces of each wafer, and then, a silicon nitride film is formed on the pad oxide film on both surfaces of the wafer by exposure to a nitriding gas atmosphere.
Subsequently, the pad oxide film and the silicon nitride film on the pad oxide film on one surface of the wafer are removed from the one surface, namely, the front surface of the wafer by patterning. Then, the silicon nitride film on the pad oxide film is removed from the other surface of the wafer with the pad oxide film remaining.
Subsequently, the wafer with the partially exposed front surface and the fully exposed reverse surface stripped of the silicon nitride film is exposed to the oxidizing atmosphere. In the oxidation, isolation regions consisting of silicon dioxide are formed on the exposed regions on the front surface of the wafer. On the reverse side, however, because the surface on this side is covered with the pad oxide film consisting of silicon dioxide, only a trace amount of silicon dioxide grows under the pad oxide film, so that the pad oxide film covering the reverse surface does not substantially increase in thickness.
After the isolation regions have been formed, the pad oxide film and the silicon nitride film remaining on the front surface are removed, and the silicon wafer, on which the isolation regions have been formed, is transferred to the subsequent manufacturing steps for forming semiconductor devices.
The pad oxide film remaining on the reverse surface of the silicon wafer serves as a protective film for the reverse surface of the silicon wafer, and when contaminants, such as a heavy metal or any other foreign substance, adhere to the pad oxide film in the subsequent steps, the pad oxide film is etched to remove the surface layer of this oxide film together with the contaminants in the manufacturing steps where the cleaning of the wafer is required. In this manner, the silicon wafer can be protected from contamination from its reverse surface.
However, the pad oxide film remaining on the reverse surface of the silicon wafer is smaller in thickness than the isolation regions formed on the front surface the wafer, and will be perished by several etching processes.
Therefore, if the etching process is carried out on the above-mentioned pad oxide film to clean the wafers in each of the manufacturing steps, the pad oxide film as a sacrifice layer is gradually abraded off, giving rise to chances for the silicon substrate under the pad oxide film to be damaged by the etching processes.
Therefore, the object of the present invention is to provide a selective oxidation method capable of forming desired isolation regions on a front surface of a silicon wafer and relatively easily forming an oxide film with a thickness sufficient to act as a sacrifice layer on the reverse surface.
To achieve the above object, the present invention adopts the following configuration.
A method for oxidizing a silicon wafer comprises the steps of:
covering each of whole areas of both surfaces of a silicon wafer by an oxidation inhibitor film with interposition of a pad oxide film;
patterning the pad oxide film and an oxidation inhibitor film on the pad oxide film on one surface of the wafer to form desired patterns to partially expose the one surface of the wafer through the patterns;
removing the pad oxide film and the oxidation inhibitor film on the pad oxide film formed on the other surface of the wafer to expose the whole area of the other surface of the wafer;
oxidizing the regions exposed partially on the one surface of the wafer and the whole area of the other surface of the wafer simultaneously to grow a silicon dioxide film on both surfaces of the wafer; and
removing the oxide inhibitor film overlying the pad oxide film and the underneath pad oxide film remaining on the one surface of the wafer.
According to the selective oxidation method according to the present invention, before the selective oxidation of the one surface of the silicon wafer, the pad oxide film and the oxidation inhibitor film on the pad oxide film covering the other surface of the wafer may be removed to expose the whole area of the other surface of the wafer.
Therefore, simultaneously with the selective oxidation of the exposed regions on the one surface, an oxide film may be grown on the whole area of the other surface. At this time, on the other surface, because of its not being covered with the pad oxide film unlike in the prior art, a pad oxide film can be grown to substantially the same thickness as on the exposed regions on the one surface. This oxide film may be used as the protective film to prevent contamination and may also be used as a sacrifice layer to clean contamination.
The oxidation inhibitor film may be a silicon nitride film.
The pad oxide films covering both surfaces of the wafer may be formed simultaneously in a batch type thermal oxidation furnace.
The silicon nitride films covering both surfaces of the wafer with interposition of the pad oxide film may be formed simultaneously by a batch type low-pressure CVD.
The exposed regions on the one surface of the wafer and the exposed area on the other surface of the wafer may be subjected to the oxidation process by a batch type thermal oxidation furnace.
The oxide film partially formed on the one surface of the wafer may be used isolation regions.
The oxide film formed on the whole area of the other surface of the wafer may be used as a sacrifice layer to remove contamination, which occurs in handling of the wafer, by an etching process.